Article having surface layer of uniformly oriented, crystalline, organic microstructures

ABSTRACT

Article comprising a substrate bearing a microlayer which comprises uniformly oriented, crystalline, solid, organic microstructures and a method for preparing said article. The microstructures may be mono- or polycrystalline. In the preferred embodiment, the microstructures are of uniform shape and size. 
     The articles can be prepared by (1) depositing a vapor of an organic compound as a thin, continuous film onto a substrate to provide a composite, and (2) annealing the composite in a vacuum sufficiently such that a physical change is induced in the orginal deposited film to form the microstructures. 
     The microlayer can be overcoated with other materials to provide desired properties to the article. The articles of this invention are useful for many forms of light trapping, energy absorption, imaging, data transmission and storage, and gradient index applications.

TECHNICAL FIELD

This invention relates to articles having microstructured surfaces.

BACKGROUND OF THE INVENTION

Microstructured surfaces have been prepared by many different types ofchemical and physical deposition processes. These processes areconveniently categorized as "wet chemical" processes and "dry chemical"processes. An example of a wet chemical process involves exposure ofaluminum, magnesium, or zinc metal film or alloy thereof to saturatedsteam or an aqueous oxidizing solution to form an oxyhydroxide boehmitemicrostructured surface (see U.S. Pat. Nos. 4,123,267; 4,190,321;4,252,843; 4,396,643). An example of a dry chemical process involvesproduction of thread-like, non-uniform, mosaic structures of randomlyoriented alpha- and beta-copper phthalocyanine crystallites byannealing, in air, films of copper phthalocyanine that had been vapordeposited in vacuum on glass and potassium chloride substrates, or byvapor deposition of copper phthalocyanine on a heated substrate (see"Thermal Behavior of Thin Copper - Phthalocyanin Films", O. Hirabaru, T.Nakadhima, H. Shirai, Vacuum (Japan) 22(7) (1979) 273).

In most applications of the aforementioned wet chemical and dry chemicalprocesses, the microstructured surfaces are generally made of metallicor inorganic materials only. In addition, the microstructured surfacecomprises a polycrystalline or amorphous material, not singlecrystalline material.

Microstructured surfaces currently in use and the processes used to makethem have one or more shortcomings. Some have wide variation in the sizedistribution or aspect ratio of the microstructure. This variationrenders the properties of the microstructured surface difficult tospecify, making the surface a poor candidate for optical uses such asabsorption or reflectivity. Some microstructures are peculiar tospecific substrates, many of which are subject to corrosion. Somemicrostructured surfaces are opaque to electromagnetic radiation. Thislimitation precludes their use with transparent media. Most of theaforementioned dry processes rely on a dynamic method of formingmicrostructures during the step of film deposition, which necessarilyinvolves either multiple deposition parameters that can be difficult tocontrol in practice or non-equilibrium growth mechanisms that areunpredictable. In addition, many of the dry or wet chemical processesare suitable for imparting microstructures to only relatively smallsurface areas during a given time period due to limitations of availableequipment.

SUMMARY OF THE INVENTION

In one aspect, this invention provides articles comprising a substrateand, overlying the surface of said substrate, a microlayer whichcomprises an array of discrete, uniformly oriented, with respect to thesubstrate surface, crystalline, solid, organic microstructures (ormicroelements), e.g. in the form of whiskers. Preferably, themicrostructures are of uniform shape and size, and comprise singlecrystals having cross-sectional dimensions less than the wavelength ofvisible light. The microstructures are densely arrayed, and themicrolayer, which has a high specific surface area, can be produced on awide variety of substrates by means of a simple process, without beingrestricted to a small surface area.

In another aspect, this invention involves a process for making the saidarticles comprising the steps of (1) depositing a vapor of an organicsubstance as a thin, continuous film onto a substrate to provide acomposite, and (2) annealing, i.e. heating, the composite in a vacuumsufficiently such that a physical change is induced in the depositedfilm to form said microstructures. The aspect ratio of themicrostructures can be controlled by varying the thickness of the filmformed in the first step of the process. Optionally, a third stepinvolves coating the resultant annealed article with a coating materialto impart a desired property thereto, such as low reflectivity, i.e.light trapping.

Organic substances that are useful for preparing the microstructuredsurfaces of this invention include compounds having planar moleculescomprising chains or rings, preferably rings, over which π-electrondensity is extensively delocalized. These compounds include polynucleararomatic hydrocarbons, such as perylenes, and heterocyclic aromaticcompounds, such as porphyrins and phthalocyanines.

In yet another aspect, the microstructures of the surface overlying thesubstrate can serve in turn as another substrate which can be coatedand, in turn, overcoated again. The resultant coated articles are thensuitable for many forms of light trapping, energy absorption, imaging,data transmission and storage, and gradient index applications,depending on the configuration of the microstructures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning electron micrograph (hereinafter SEM at 30,000magnification (30,000×) taken at normal incidence to the microlayer of aperylene red film that had first been vapor deposited onto an aluminumsubstrate and then vacuum annealed.

FIG. 2 shows a SEM at 30,000× of the microlayer of FIG. 1 taken at anangle of incidence of 45° to the composite surface. The whisker-shapedmicrostructures are about 1.5 micrometers long, uniform in cross-sectionalong their lengths, discrete, non-contacting, and orientedsubstantially perpendicular to the substrate, which is made of aluminum.(Variations from the perpendicular were induced by the act of preparingthe microstructures for the photomicrograph.) The microstructures appearto have a round cross-section because of the limiting resolution of thescanning electron microscope.

FIG. 3 shows a SEM at 30,000× taken at an angle of incidence of 45° tothe composite surface of a rough portion of the initial perylene redfilm that had been vapor deposited on an aluminum substrate, but notvacuum annealed.

FIG. 4 is an infrared (IR) transmittance spectra taken in an externalreflectance configuration demonstrating IR absorption band intensitychanges that can be used to monitor the growth of microstructures andindicate degree of completeness of microstructure growth.

FIG. 5 shows a transmission electron micrograph (hereinafter TEM) at280,000× taken at normal incidence to the surface of platinumshadow-coated perylene red microstructures. The microstructures are lathshaped, with very straight edges, are uniform in cross-section alongtheir lengths, and are uniform from microstructure to microstructure.The pattern of dots are clusters of platinum microislands vapordeposited as a coating onto the microstructures at an angle of incidenceof 45°, after they were transferred to a polyvinyl formaldehyde-coatedcopper TEM sample grid. The platinum preferentially nucleated at defectsites on the surface of the microstructures. The platinum very clearlydelineates the presence of single crystal step edges running parallel tothe long dimensions of the microstructures, which indicates a highdegree of crystalline perfection along the entire length of themicrostructures.

FIG. 6 shows a SEM at 10,000× taken at an angle of incidence of 45°wherein a mass equivalent of 500 Angstroms of copper was coated bysputter deposition onto the microstructures. The coated article provideda very efficient light-trapping or absorbing surface, and, accordingly,appeared very black.

FIG. 7 shows a SEM at 10,000× taken at an angle of incidence of 45°wherein a mass equivalent of 1000 Angstroms of copper was coated bysputter deposition onto the microstructuees. The surface of the coatedarticle appeared dark gray. The surface had an extraordinarily lowspecular reflectivity.

FIG. 8 shows a SEM at 10,000× taken at an angle of incidence of 45°wherein a mass equivalent of 2000 Angstroms of copper was coated bysputter deposition onto the microstructures. The surface of the coatedarticle appeared light gray. The surface had an extraordinarily lowspecular reflectivity. The scale of the microlayer was seen to change onaccount of the increase in epitaxial metal growth on the sides of themicrostructures.

FIG. 9 shows a SEM at about 5,000× taken at an angle of incidence of 45°showing a vapor deposited polyethylene film deposited as an overcoatingover a perylene red microlayer after the perylene red microstructureshad been previously sputter coated with 500 Angstroms of copper. Aportion of the polyethylene overcoating was stripped away to reveal theunderlying copper metallized microstructures.

FIG. 10 shows a schematic diagram of a vacuum deposition apparatus thatcan be used in the first step of the process of this invention.

DETAILED DESCRIPTION

The article of this invention comprises a substrate bearing on at leastone major surface thereof a microlayer comprising an array of discrete,single- or poly-crystalline, uniformly oriented (with respect to thesubstrate surface) microstructures of a solid, organic material.

As used herein, the term "microstructure" means the smallest individualrepeating unit of a microlayer. The term "microlayer" means the layerformed by all the microstructures taken together.

The chemical composition of the microstructures will be the same as thatof the starting organic material. Organic materials that are suitablefor the practice of the present invention include planar moleculescomprising chains or rings over which x-electron density is extensivelydelocalized. Organic compounds that are suitable for use in thisinvention generally crystallize in a herringbone configuration.Compounds that are preferred for thi invention can be broadly classifiedas polynuclear aromatic hydrocarbons and heterocyclic compounds.Polynuclear aromatic compounds are described in Morrison and Boyd,Organic Chemistry, Third Edition, Allyn and Bacon, Inc. (Boston: 1974),Chapter 30, and heterocyclic aromatic compounds are described inMorrison and Boyd, supra, Chapter 31. Among the classes of polynucleararomatic hydrocarbons preferred for this invention are naphthalenes,phenanthrenes, perylenes, anthracenes, coronenes, pyrenes, andderivatives of the compounds in the aforementioned classes. A preferredorganic material is commercially available perylene red pigment,N,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide), hereinafterreferred to as perylene red. Among the classes of heterocyclic aromaticcompounds preferred for this invention are phthalocyanines, porphyrins,carbazoles, purines, pterins, and derivatives of the compounds in theaforementioned classes. Representative examples of phthalocyaninesespecially useful for this invention are phthalocyanine and its metalcomplexes, e.g. copper phthalocyanine. A representative example ofporphyrins useful for this invention is porphyrin. The organic materialsare preferably capable of forming an initially continuous film of athickness of at least several hundred Angstroms to a few thousandAngstroms.

Each individual microstructure is single- or poly-crystalline ratherthan amorphous. The thin film microlayer is highly anisotropic due tothe crystalline nature and uniform orientation of the microstructures.

The orientation of the microstructures in relation to the surface of thesubstrate is generally uniform. As used herein, the term "uniform", withrespect to orientation, means that the angles between an imaginary lineperpendicular to the surface of the substrate and the major axes of atleast 90% of the microstructures varies no more than approximately 5°from the mean value of the aforementioned angles. The microstructuresare usually oriented perpendicular or normal to the substrate surface,the surface normal direction being defined as that direction of the lineperpendicular to an imaginary plane lying tangent to the local substratesurface at the point of contact of the base of the microstructure withthe substrate surface. The surface normal direction is thus seen tofollow the contours of the surface of the substrate. Surfacenormal-oriented microstructures render the microlayer capable of a highlevel of light trapping arising from multiple scattering of incidentlight between the surfaces of the individual microstructures.

The size, i.e. length and cross-sectional area, of the microstructuresare generally uniform from microstructure to microstructure. As usedherein, the term "uniform", with respect to size, means that the majordimension of the cross-section of the individual microstructures variesno more than about 23% from the mean value of the major dimension andthe minor dimension of the cross-section of the individualmicrostructures varies no more than about 28% from the mean value of theminor dimension. The uniformity of the microstructures providesuniformity in properties, and performance, of articles containing themicrolayer. Such properties include optical, electrical, and magneticproperties. For example, electromagnetic wave absorption, scattering,and trapping are highly dependent upon uniformity of the microlayer.

Although microstructures can have varieties of shapes, in any one givenmicrolayer, the shapes of the individual microstructures are preferablyuniform. Shapes include rods, cones, cylinders, and laths. In thepreferred embodiment, the microstructures are lath-shaped whiskers. Themicrostructures can have a large aspect ratio, generally ranging fromabout 10 to about 100. As used herein, the term "aspect ratio", withrespect to a microstructure, means the ratio of the length (majordimension) to the diameter or width (minor dimension) of themicrostructure. Furthermore, it is relatively simple to control theaspect ratio by specifying the thickness of the initially depositedorganic film. One advantage of being able to control the aspect ratio isthat the microlayer can be given variable anisotropic optical propertiestailored for a desired function. The major dimension of themicrostructures is directly proportional to the thickness of theinitially deposited film. Because it is clear that all the originalorganic film material is being converted to microstructures, and becausethe microstructures have uniform cross-sectional dimensions, arediscrete, and are separated by distances on the order of their width,conservation of mass implies the lengths of the microstructures will beproportional to the initially deposited film thickness. Hence thelengths and aspect ratios of the microstructures can be variedindependently of their cross-sectional dimensions and areal densities.As used herein, "areal density" means the number of microstructuresgrown per unit area. For example, it has been found that length ofmicrostructures are approximately ten times (10×) the thickness of avapor deposited film, when the thickness ranges from about 0.05 to about0.2 micrometers. The minor dimension of the microstructures isdetermined by the surface free energy ratios of the boundingcrystallographic side planes and can be explained by Wulff's theorem.The surface area of the microlayer of the article of this invention,i.e. the sum of the surface areas of the individual microstructures, ismuch greater than that of the organic film initially deposited on thesubstrate. This feature allows the article to function effectively as alight trapping medium.

The spectral absorption characteristics of the microstructures aresubstantially similar to those of the starting organic material, unlessthey are overcoated. The index of refraction of the microstructures willbe intermediate between that of a solid film of the organic material andthat of the surrounding medium, due to the discrete nature of themicrostructures; that is, there is a gradient index of the typedescribed in Lee and Debe, "Measurement and Modeling of theReflectance-Reducing Properties of Gradient Index MicrostructuredSurfaces", Photographic Science and Engineering, Vol. 24, No. 4,July/August 1980, pp. 211-216.

Substrates that are useful in the practice of this invention can beselected from those materials which will maintain their integrity at thetemperatures and vacuums imposed upon them during the vapor depositionand annealing steps. The substrate can be flexible or rigid, planar ornon-planar, convex, concave, aspheric, or combinations thereof.Materials such as ceramics, e.g. glass, metal, metal oxides, or theirmixtures can be used as substrates. Organic, polymeric materials able towithstand annealing temperatures can also be used. Representativeexamples of metals useful as substrates for this invention includealuminum, cobalt, copper, molybdenum, nickel, platinum, and tantalum.The varying nature of metals or metal oxides brings about no observabledifferences in the final composite, i.e., both the pure metals andmetals having oxide coatings serve as inert substrates. Metal substratescan thus be exposed to the atmosphere before coating a film of organicmaterial thereon without adverse affects. Thickness of the substrate canvary.

A surface microlayer such as that depicted in FIGS. 1 and 2 can beformed according to the following procedure. A clean metal substrate isprepared by either vacuum vapor depositing or ion sputter depositing ametal film onto a previously cleaned float glass slide. Suitable metalsare those that are useful for preparing substrates, as noted previously.The thickness of the metal film is preferably on the order of 1000Angstroms. Other materials useful for forming the substrate are barefloat glass and standard laboratory microscope slides cleaned accordingto usual laboratory procedures. The substrate can be coated with thelayer of organic material by depositing by means of physical vacuumvapor deposition, i.e. sublimation of the organic material under anapplied vacuum. The temperature of the substrate during vapor depositionis not critical and the temperature range chosen can be varied,depending upon the organic material selected. For perylene red, asubstrate temperature near room temperature (25° C.) is satisfactory.The rate of vacuum vapor deposition can be varied. Thickness of thelayer of organic material deposited can vary and the thickness chosenwill determine the major dimension of the resultant microstructuresafter the annealing step is performed. Layer thicknesses in the range of0.05 to 0.25 micrometers are generally preferred.

A typical apparatus for conducting vacuum vapor deposition is shownschematically in FIG. 10 and comprises a bell jar 10, a vacuum baseplate 12 with ports 14, 16, 18 for rotary motion means 20, electricalpower means 22, and water cooling means 24, respectively, quartz crystalthin film deposition monitor 26, a shutter 28 operated by the rotarymotion means 20, a crucible 30 for the organic source material 32, atungsten wire basket 34 for supporting and resistively heating thecrucible 30. These apparatus are known to those of ordinary skill in theart and are commercially available.

Electrical power can be used to heat an alumina or quartz sourcecrucible 30 containing the organic material 32 to be vapor deposited.The means for providing rotational motion 20 allows moving a shutter 28,so as to either interrupt or permit the deposition of the organicmaterial 32 onto the substrate 36, which is mounted above the sourcecrucible 30.

The substrate 36, such as previously described, is placed in the vacuumbell jar 10 to which is attached a liquid nitrogen trapped, oildiffusion pump (not shown). The substrate 36 is positioned, metal sidedown, above the source crucible 30 within the bell jar 10. The distancefrom the substrate 36 to the source crucible 30 will affect thedeposition rate and the film thickness. A typical distance isapproximately 16.5 cm. The crucible 30 is first cleaned by heating itempty to glowing red temperatures under vacuum. The organic material 32,e.g. perylene red, is placed in the bottom of the crucible 30 to a depthof about 2 mm and gently tapped down to eliminate air pockets. Afterevacuating the bell jar 10 to a pressure below 1×10⁻⁶ Torr, the crucible30 is heated to 260° C. for 30 minutes as a "pre-soak" to degas theorganic material 32. A movable shutter 28 between the source crucible 30and substrate 36 should remain closed during the pre-soak phase. Thepre-soak step is then continued at about 285° C. for an additional 45minutes. During this time the system pressure is maintained at ca.8×10⁻⁷ Torr. Heating current to the tungsten basket 34 holding thesource crucible 30 is then increased to bring the crucible temperatureto 380° C. to 420° C. After 31/2 minutes, the shutter 28 is opened,allowing the organic material to sublime and deposit on the substrate.At any given crucible temperature, the deposition rate is greatest whenthe organic material 32 uniformly wets the interior of the crucible 30and is prevented from forming an iris-like deposit on the top rim of thecrucible by uniform heating. Deposition rates typically vary from 20Angstroms per minute to 400 Angstroms per minutes and film thicknessestypically range from 500 to 2000 Angstroms, but these parameters canvary widely. During vapor deposition, no special means need be used tocontrol the substrate temperature. It is typically found that thetemperature of the metallized glass slide increases 10° to 15° C. aboveambient during vapor deposition.

The rate of sublimation, and consequently, thickness of the organic filmlayer deposited on the substrate 36 can be determined by the use of aquartz crystal oscillator thin film deposition monitor 26. Such monitorsare commercially available and are well known in the art.

In the annealing step, the coated substrate from the vapor depositionstep is heated in a vacuum for a sufficient period of time such that thedeposited film layer undergoes a physical change resulting in productionof a microlayer comprising pure, single- or poly-crystallinemicrostructures. Exposure of the organic material coated substrate tothe atmosphere before the annealing step is not detrimental tosubsequent microstructure formation.

In the annealing step for films of perylene red and copperphthalocyanine, the coated substrate can be heated by any suitable means(not shown) in a vacuum (not shown) of at least moderate quality, e.g.,1×10⁻³ Torr or lower, and in a temperature range of 160° C. to 230° C.The time of annealing is dependent on the annealing temperature but arange of about 1/2 to about 6 hours, preferably about 11/2 to about 4hours, is generally sufficient to convert the original organic filmlayer on the substrate to the microstructured film, as can be determinedby scanning electron microscopy and reflection absorption infraredspectroscopy (see M. K. Debe, Appl. Surface Sci., 14 (1982-83) pp.1-40).

The interval between the vapor deposition step and the annealing stepcan vary from several minutes to several months, with no significantadverse effect, as long as the coated composite is stored in a coveredcontainer. However, the length of the interval is not critical. Theannealing step can be monitored in situ with either an infraredspectroscopic technique (see FIG. 4), or the reflectance of a He-Nelaser beam at near grazing incidence. As the microstructures develop,the infrared band intensities change and the laser specular reflectivitydrops, thus allowing the conversion to be carefully followed. Thesubstrate is allowed to undergo a natural, unaided cooling before thevacuum chamber is backfilled with a gas (e.g., air) to attainatmospheric pressure and the substrate removed from the vacuum chamber.

Only controlled vacuum deposition and vacuum annealing are necessary togenerate the articles of this invention. Heretofore, microstructuredsurfaces have required vacuum processes such as plasma, sputter, orreactive ion etching, or non-vacuum processes such as chemical etching,electro- or electroless deposition, anodizing, and etching for theirproduction. Furthermore, the composition of the microstructuresheretofore were amorphous or polycrystalline oxides, alloys, or otherpolymeric forms which lacked the oriented, pure crystalline propertiesof the articles of this invention.

Other embodiments of this invention include articles that can be made byprocesses comprising additional coating of the articles obtained fromthe annealing step.

"Coating" means that another material, organic, or inorganic, is either(a) put in intimate contact with the microstructures of the annealedarticle, or (b) applied as an overlayer to `blanket` the annealedarticle. In the first case, the coating can come in intimate contactwith essentially the entire surface of the microstructure. Furthermore,this coating may or may not be a continuous film, depending on themicrostructures and overall microlayer. In the second case, the coatingcomes in contact primarily with the tips or distal ends of themicrostructures so as to provide a continuous film. The successiveovercoating of a previously coated article is also within the purview ofthis invention.

The coating of the annealed article is preferably done by a vacuumdeposition process to avoid the disturbance of the microstructures bymechanical-like forces of contact. This vacuum deposited coating canserve to strengthen the microlayer and permit further overcoating bymeans of the vacuum deposition technique described in this invention orby means of conventional coating techniques such as dipping, spraying,roll coating, knife, blade coating, and the like. Overcoating the coatedand annealed articles with organic or inorganic materials can provide anovercoat as continuous or discontinuous films, depending upon themicrolayer of the coated article.

Marked changes in the resultant coated article can be noted. Forexample, coating an annealed article with copper provides a blackcomposite of low specular reflectivity and changed microlayer.Polyethylene overcoating of an annealed article, previously coated withcopper, provides a composite with secondary microtextured surface havingdifferent physical and chemical properties, the details of which aredescribed in the examples.

The articles of this invention are useful for many forms of lighttrapping, energy absorption, imaging, data transmission and storage andgradient index applications. The articles of this invention can be usedto prepare photovoltaic devices, such as the type of device described inU.S. Pat. No. 4,252,865, radiation absorbing devices, e.g. selectivesolar absorbers, flat plate solar collectors, solar absorption panels,such as the type of device described in U.S. Pat. No. 4,148,294, solarcells, such as the type of device described in U.S. Pat. No. 4,155,781,photo absorbing surfaces, such as the type of device described in U.S.Pat. No. 4,209,008, optical storage media, such as the type of devicedescribed in "Textured surfaces: Optical Storage and OtherApplications", H. G. Craighead, R. E. Howard, J. E. Sweeney, and D. M.Tennant, J. Vac. Sci. Technol., 20(3), March 1982.

This invention is further illustrated by the following, non-limitingexamples.

EXAMPLE 1

This example involves an article comprising an aluminum substrate thatbears on the surface thereof a microlayer of perylene red. A substratewas formed by vacuum sputter depositing an aluminum film onto a 3 mm×30mm×10 cm glass slide. The aluminum film was allowed to air oxidize bynatural exposure to the atmosphere. Then, the aluminum coated glassslide was installed in a vacuum bell jar to which was attached a liquidnitrogen trap and diffusion pump capable of attaining vacuums in therange of about 1×10⁻⁷ Torr. Approximately 0.1 gram of commerciallyavailable perylene red was placed in an alumina crucible, which was inturn heated by a tungsten basket heater in the vacuum bell jar. Theperylene red was vacuum purified by degassing it during a "pre-soak"heating period, during which time the crucible was maintained at 260° C.to 385° C. for over 2 hours. A shutter positioned between the crucibleand the aluminum coated glass slide, and located 16.5 cm (6.5 inches)above the source crucible, was kept closed during the pre-soak period.Immediately prior to the start of deposition of perylene red onto thealuminized substrate, the electrical power applied to the crucibleheating basket was increased to cause the interior crucible temperatureto exceed 380° C., thus initiating sublimation of the perylene red.

The sublimation rate of the perylene red was monitored with a quartzcrystal oscillator thin film deposition monitor. The shutter was openedto allow the perylene red vapor to deposit on the surface of thealuminum substrate and thin film monitor. The perylene red film wasallowed to form on the aluminum substrate to a total thickness of 0.15micrometers before the shutter was closed. During the deposition, thealuminized substrate temperature was not actively controlled but variedbetween 25° to 40° C. FIG. 3 shows an SEM micrograph of a rough portionof the as-deposited film, which, after the annealing step resulted inthe microstructured surface (see FIGS. 1 and 2).

The aluminized glass composite, bearing a 1500 Angstrom thick film ofperylene red, was removed from the bell jar system, and inserted into asecond similar vacuum bell jar equipped with a heater assembly capableof heating the entire continuous perylene red-coated surface of thecomposite by thermal conduction through the 3 mm thick glass substrate.The perylene red composite was heated at approximately 190° C. over aperiod of several hours while the annealing step was continuouslymonitored, with the technique of reflection absorption infraredspectroscopy (RAIR) to measure the phase transition growth of themicrostructures. FIG. 4 shows the observed infrared spectral changesthat occurred in the composite before and after the formation ofmicrostructures. The significant relative band intensity changes areindicative of the occurrence of preferential orienting of perylene redmolecules relative to the aluminum substrate.

Microstructures 40 (see FIG. 2) are seen to be about 1.5 micrometers inlength, oriented substantially perpendicular to the aluminum filmsubstrate 42, and, as may be deduced from inspection of FIG. 1, haveareal number densities on the order of 50 per square micrometer.Microstructures 40 appear to have uniform cross-sectional dimensionsalong their lengths, and though touching at times at their bases, areessentially discrete and non-contacting. While they appear to have around cross-section in FIGS. 1 and 2, due to limited resolution of theSEM, the microstructures actually are lath shaped, as shown in FIG. 5.Microstructures 44 in the TEM image are seen to have very straight sidesand are exceptionally uniform in cross-section along their lengths. Thepattern of dots 46 are clusters of platinum microislands that were vapordeposited onto the microstructures after the latter were mechanicallyremoved from the substrate by rubbing and transferred to a Formvar®brand polyvinylformaldehyde plastic coated TEM sample grid. The platinumpreferentially nucleates at defect sites on the surfaces of themicrostructures and very clearly delineates the presence of step edges48 running parallel to the major dimension the microstructures (see FIG.5), indicating a high degree of crystalline perfection along the entirelength of the microstructure.

Microstructures of perylene red produced by the process of thisinvention are single- or polycrystalline, have a cubic lattice structureand [211] growth axis standing substantially normal to the surface ofthe substrate. Cross-sectional dimensions of perylene redmicrostructures are uniform along their lengths and display a narrowsize distribution from microstructure to microstructure. The width ofperylene red microstructures is 0.052 micrometers on average and theirthickness is 0.027 micrometers on average. Side planes of perylene redmicrostructures have (001) and (111) crystal indices. When the initiallydeposited film is fully converted to the microstructure form, theindividual, oriented microstructures are discrete, essentiallynon-touching at their base (i.e., the point of contact of themicrostructure with the substrate), and are densely arrayed with meanspacings on the order of 0.05 micrometers, giving areal number densitiesof about 5 billion microelements per square centimeter or 50microelements per square micrometer. Average length of themicrostructures depends on the initial uniform film thickness, and isapproximately 10 to 15 times this initial value. Aspect ratios aretypically in the range of 10 to 80 for the initial thickness range aboveand an order of magnitude increase in specific surface area.

EXAMPLE 2

This example involves another embodiment of this invention, wherein themicrolayer of a microstructured article made according to the method ofExample 1 was coated with copper metal.

A mass equivalent of 500 Angstroms of copper was ion sputter depositedonto the microstructures 50 of the article made according to Example 1(see FIG. 6); this amount was increased to 1000 Angstroms formicrostructures 52 (see FIG. 7) and 2000 Angstroms for microstructures54 (see FIG. 8). Metal coated microstructures 50 provide a veryefficient light trapping surface and accordingly appear very black. Themetal coated microstructures 52 and 54 caused the surface to appear darkgray and light gray respectively, and provided the surface with anextraordinarily low specular reflectivity. The microlayer scale was seento change as the dimensions of the microstructures changed because ofthe epitaxial metal growth on the sides of the microstructures.

EXAMPLE 3

This example involves another embodiment of this invention wherein a 500Angstrom coating of copper metal, which had been previouslysputter-coated onto a microstructured article made according to Example1, is overcoated with a film of polyethylene by means of vacuum vapordeposition (see FIG. 9).

The polyethylene was deposited onto the copper coated article to athickness substantially greater than the length of the major dimensionof the microstructures 56, so as to generate a new microtextured surfaceof much larger scale and different chemical nature. The polymerovercoating may also serve as a protective coating for the underlyingmicrostructured surface.

EXAMPLE 4

This example involves an article having copper deposited as a coatingonto a microstructured article made according to Example 1, wherein aplatinum substrate had first been deposited onto a glass slide by vapordeposition.

As described in Example 1, a glass slide sputter coated with platinum,was coated by means of vapor deposition with perylene red to a thicknessof 1650 Angstroms at a mean deposition rate of 410 Angstroms per minute.The annealing step was carried out by heating the sample at graduallyincremented temperatures in the range of ca. 165° to 205° C. Theannealing step was monitored with IR spectroscopy until the bandintensities became constant with time, i.e. stabilized.

In another vacuum system, copper was then ion sputter coated onto themicrostructured surface of the article in an argon atmosphere at apressure of 5 mTorr and 500 watts output power from a 13.6 megaHertz RFpower supply. The article was masked so as to produce three areas ofincreasing thickness of the copper coating, which was calibrated to beequivalent to uniform film thicknesses of 500, 1000 and 2000 Angstroms,respectively. After the coating step was complete, the sample regionhaving a 500 Angstrom layer of sputtered copper appeared jet black inordinary light due to the high light trapping properties of the region.The region having a 1000 Angstrom layer of copper appeared dark gray andthe region having a 2000 Angstrom layer appeared light gray. It was alsoobserved that the particular thickness of metal coating needed toproduce a desired level of grayness depended on the metal used.

Metal coated microstructures of this invention had an extremely lowspecular reflectivity. For example, the specular reflectivity of thesample region having 1000 Angstrom thick layer of copper was measured at7.5° off normal incidence and found to be less than 1.5% betweenwavelengths of 2500 nm and 2000 nm, and below 0.5% between wavelengthsof 2000 nm and 185 nm, relative to an aluminum reference mirror. Thisability to go from a surface characterizable as having low specular andlow diffuse reflectivity to one of low specular and high diffusereflectivity by simply doubling the thickness of metal coating on themicrostructures is considered an important aspect of this invention.

EXAMPLE 5

In this example, the metal-coated article prepared in Example 2 wasovercoated, by vacuum vapor coating, with a non-conductive, polymericmaterial, namely polyethylene. Commercially available polyethylenepellets, were inserted into a quartz tube, approximately 13.3 cm (5.25inches) long and 0.95 cm (0.35 inches) in diameter, having a 0.16 cm by8.26 cm slot cut longitudinally on one side of the tube. The tube wasplaced in a vacuum bell jar and heated by passing current through anichrome film previously sputter coated on the outside surface of thequartz tube. The ends of the quartz tube were blocked with quartz plugs.The slot was oriented upward in the vacuum bell jar and parallel to thelong axis of the metal-coated microstructured article, which waspositioned approximately 3.8 cm. (1.5 inches) above the quartz tube. Thequartz tube was heated by dissipating ca. 9 watts of power for 16minutes in the nichrome film, which caused a film of polyethylene to bevapor coated onto the metal coated microstructures of the composite.FIG. 9 shows an SEM micrograph of the sample, at a point where theovercoated polyethylene 58 was stripped away, revealing the underlyingmetal coated microstructures 56 in a region where 500 Angstrom thicklayer of copper had been deposited. A SEM micrograph of the underside ofthe stripped away polyethylene overlayer revealed a multiplicity ofhemispherical depressions where the distal ends (rounded `heads`) of thecopper-coated microstructures had been partially embedded in thepolyethylene overlayer. The visual effect of the polyethylene was toreduce even further the specular and diffuse reflectivity of thesurface. For example, the region with the 1000 Angstrom thick layer ofcopper and polymeric coating, discussed in Example 4 above, was nowmeasured to have a specular reflectivity at 7.5° off normal incidence ofonly 0.9% at a wavelength of 2500 nm, reducing to 0.1% at wavelengthsbetween 2000 nm and 900 nm, and 0.3% at a wavelength of 850 nm.

EXAMPLE 6

This example demonstrates generation of a microlayer wherein the organicmaterial is a porphyrin.

Metal-free phthalocyanine was deposited onto an aluminized glasssubstrate, according to procedure of Example 1. The thickness of theorganic layer was 500 Angstroms, the deposition rate was 45 Angstromsper minute, and the background pressure was ca. 2×10⁻⁶ Torr. Theresulting composite was installed into a second vacuum chamber andannealed at a pressure of 4×10⁻⁸ Torr by gradually increasing the filmtemperature in steps to a maximum of approximaely 190° C. over a periodof 21/2 hours. Heating was stopped when the maximum temperature wasreached. Scanning electron micrographs of the resulting surface showedan array of crystalline, whisker-shaped microstructures, having auniform cross-section, approximately square-shaped, each side of thesquare being approximately 500 Angstroms. The microstructures wereoriented essentially perpendicular to the surface of the substrate, andthey had an aspect ratio in the range of 5 to 10. The number ofmicrostructures per unit area was not as high as that for the perylenered material in the previous examples, probably due to prematurestoppage of the annealing.

A second sample of the same organic material was deposited and annealedin the same manner as was the first sample, the only differences beingthat the layer of organic material was 900 Angstroms thick, and theannealing was maintained above ca 180° C. to 200° C. for over two hours.During the two hour period at elevated temperature, some of the materialdesorbed, but the material remaining on the cooler edges of the sampledisplayed the same oriented crystalline microstructures as seen on thepreviously described 500 Angstrom sample. The microstructures wereslightly longer and the areal number density was slightly higher, beingabout 5 microstructures per square micrometer.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

What is claimed is:
 1. An article comprising a substrate bearing on atleast one major surface thereof a microlayer comprising an array ofdiscrete, uniformly oriented, crystalline, solid microstructures, saidmicrostructures comprising an organic compound wherein the moleculethereof is planar and comprises chains or rings over which π-electrondensity is extensively delocalized.
 2. An article according to claim 1wherein said organic compound is selected from the group consisting ofpolynuclear aromatic hydrocarbons and heterocyclic aromatic compounds.3. An article according to claim 2 wherein said heterocyclic aromaticcompound is selected from phthalocyanines and porphyrins.
 4. An articleaccording to claim 3 wherein said aromatic compound is metal-freephthalocyanine.
 5. An article according to claim 1 wherein saidmicrolayer had been formed from a layer of organic material deposited onsaid substrate.
 6. An article according to claim 5 wherein the length ofsaid microstructures is directly proportional to the thickness of saidlayer of deposited organic material.
 7. An article according to claim 1wherein said article is flexible.
 8. An article according to claim 1wherein said article is rigid.
 9. An article according to claim 1wherein said article is shaped.
 10. An article according to claim 1wherein said microstructures are oriented such that their major axes arenormal to the substrate.
 11. An article according to claim 1 whereinsaid microstructures are whiskers.
 12. An article according to claim 11wherein said whiskers have an aspect ratio which ranges from about 10 toabout
 80. 13. An article according to claim 11 wherein said whiskershave uniform cross-sectional dimensions along their major dimension. 14.An article according to claim 11 wherein said whiskers have areal numberdensities of about 50 per square micrometer.
 15. An article according toclaim 11 wherein the surface area of said whiskers is equivalent toabout ten to about fifteen times the planar surface area of saidsubstrate.
 16. An article according to claim 1 further having a firstoverlayer coated thereon.
 17. An article according to claim 16 whereinthe material of said first overlayer is inorganic.
 18. An articleaccording to claim 17 wherein the material of said first overlayercomprises a metal.
 19. An article according to claim 16 wherein thematerial of said first overlayer is organic.
 20. An article according toclaim 16 wherein said first overlayer is in intimate contact with saidmicrostructures.
 21. An article acording to claim 16 wherein saidmicrostructures are of such a shape that they have distal ends and saidfirst overlayer is primarily in contact with the distal ends of themicrostructures so as to provide a continuous film.
 22. An articleaccording to claim 16 further having a second overlayer coated over saidfirst overlayer.
 23. An article according to claim 22 wherein thematerial of said second overlayer is organic.
 24. An article accordingto claim 22 wherein the material of said second overlayer is inorganic.25. An article according to claim 24 wherein the material of said secondoverlayer comprises a metal.
 26. An article comprising a substratebearing on at least one major surface thereof a microlayer comprising anarray of discrete, uniformly oriented, crystalline, solidmicrostructures, said microstructures comprising an organic compoundselected from perylenes wherein the molecule thereof is planar andcomprises chains or rings over which π-electron density is extensivelydelocalized.
 27. An article according to claim 26 wherein said organiccompound is N,N'-di-(3,5-xylyl)perylene- 3,4:9, 10 bis(dicarboximide).